1. Field of the Invention
The invention relates to a method of fabricating multilevel interconnects of a semiconductor device, and more particularly to a method of improving the etching process of metallization.
2. Description of the Related Art
In many semiconductor circuits with a high integration, a plug is formed as a multilevel interconnect in an integrated circuit, or an interconnect between devices. In a conventional process, after forming an inter-metal dielectric layer on a first wiring layer, a via hole is formed to penetrate through the inter-metal dielectric layer and the first wiring layer is exposed. A metal layer is formed on the inter-metal dielectric layer and fills the via. The metal layer is then etched back to form a plug for interconnection.
Due to the high melting point, the high thermal expansion coefficient equivalent to silicon, a moderate internal stress, and a very good step coverage by chemical vapour deposition, tungsten is a very common material used to form a plug. A process with etch back is commonly adapted to form a tungsten plug for multilevel interconnection in the industry.
A conventional etch back process of fabricating a tungsten plug includes two steps. The first one is a main etch step, the second one is an over etch step. The main etch is performed by dry etch after the deposition of the metal layer. The unwanted metal layer on the surface of the inter-metal dielectric layer is removed by main etch. Over etch is performed to remove the residue metal layer after the main etch to ensure that the unwanted metal layer is removed completely. A conventional method of forming a tungsten plug with etch back is introduced with the reference of FIG. 1A to FIG. 1E as follows.
Referring to FIG. 1A, a dielectric layer 102 is formed on a substrate 100 which comprises a metal oxide semiconductor (MOS) device and a first metal layer. Referring to FIG. 1B, by covering a photo-resist layer on the dielectric layer 102, using photolithography and etching, a via hole or a contact window 104 for connecting different metal layers is defined, and the first metal layer is exposed. Referring, FIG. 1C, a thin metal glue layer 106, such as a titanium layer, is formed on the exposed first metal layer and the dielectric layer 102. Using physical vapour deposition, a barrier layer 108, such as a titanium nitride layer, having a thickness of 800 .ANG. to 1200 .ANG. is formed on the metal glue layer 106. A refractory and good conductive second metal layer 100, for example, a tungsten layer is formed on the barrier layer 108 and fills the via hole 104. Referring to FIG. 1D, using the barrier layer 108 as an etch stop, the second metal layer 110 is etched back to form a metal plug 110a. Referring to FIG. 2, the etch back process normally includes two steps, a step of main etch 200 and a second step of over etch 202.
After the formation of the second metal layer, using a mixture of gases containing fluorine, for example, carbon fluoride and oxygen (CF.sub.4 /O.sub.2), nitrogen triofluoride and oxygen (NF.sub.3 /O.sub.2), or sulfur fluoride and oxygen (SF.sub.6 /O.sub.2) as a reacting gas source, main etch, such as dry etch, is performed with the barrier layer 108 as an etch stop. The second metal layer 110 is etched back, and most of the second metal layer 110 on the dielectric layer 102 is removed to form a metal plug. Over etch is then performed to remove the metal residue on the dielectric layer 102. The metal layer to be etched is thus removed completely.
Referring to FIG. 1E, with the gas source during dry etch, both etch reaction and polymerised reaction occur. Etch reaction is a reaction between metal and the plasma particles produced by the gas source. The production of the etch reaction is a volatile gas. Thus, after dry etch, the metal layer is transformed into a volatile gas and vaporised from the surface. The particles produced by the gas source are reacted with each other and polymerised into polymer 114. The polymer 114 is then deposited over the substrate 100. The main reaction is the etch reaction during dry etch. Therefore, after dry etch, most of the second metal layer 110 on the dielectric layer 102 is removed, but some metal residue is left and covered by the polymer 114. An over etch is then performed to remove the metal residue for about 20 seconds to 30 seconds. However, since the residue metal is covered by the polymer 114, a very long time, for example, 20 seconds to 1 minute, or more than 1 minute, is consumed for over etch. Consequently, the barrier layer 108, the metal glue layer 106, and the tungsten plug are damaged by the very time consuming over etch process. The metal plug loss caused during over etch process may cause an open contact while connecting other metal layer.
For a structure having an uneven surface, a stringer is formed due to the ragged topography during main etch. For example, referring to FIG. 3, on a substrate 300 having a bird's beak structure 301, a stringer or a residue 304 is formed. Similarly, the stringer 304 is difficult to remove completely, even by over etch.